The present invention relates to echo cancellation in telecommunications. In particular, this invention addresses the problems of severe acoustic echo in cellular telephone technology.
An echo cancellation device is a necessary component in modem telecommunications. In situations of imperfect impedance matching, a signal broadcast over a telecommunications network may reflect back to the generator of the signal. Since the reflected signal must travel from the location of impedance mismatching and back to the generator, the generator of the signal receives the reflected signal with a delay. This return of a reflected signal with a delay is commonly known as echo. Without echo cancellation, a telephone user would not be able to effectively communicate because the reflected signal would return the speaker""s voice to him or her after each uttered sound.
In addition to echo created by impedance mismatches in the telecommunications network, the acoustics of using the phone may create echo. Acoustic echo is most commonly created by the use of a hands-free phone. In instances where the echo signal level is close to or greater than the generated signal, severe acoustic echo has been observed. In general, for echo cancellers to perform satisfactorily, the power of the echo generating signal must be greater than the echo signal by a predetermined set threshold. This threshold is commonly set at 6 dB. Thus, in situations when the echo signal power is closer to the echo generating signal power by less than the predetermined set threshold, the echo is referred to as severe echo.
In cellular telephone technology, echo cancellation devices have the ability to cut or clip the signal heard by the generator to eliminate residual echo from the echo canceller. This is typically known as a center clipping function. While prior art echo cancellers have a center clipping function, the presence of high output residual echo power prevents the echo canceller from sufficiently combating severe echo, especially severe acoustic echo.
Thus there is a particular need for a method and apparatus that detects severe echo and directs the echo canceller to utilize a center clipping function. Briefly, in accordance with an embodiment of the present invention, a system and method is provided that detects severe echo by converting an echo canceller""s adaptive filter coefficients into the frequency domain. Maximum frequency bin power is determined based on the converted coefficients and compared to a predetermined threshold. Sustained presence of maximum frequency bin power exceeding the threshold is indicative of severe echo.